This relates to the simultaneous removal of naphthenic acids and the recovery of diluents from heavy crude oil streams that were blended with diluents for transportation or extraction and production purposes.
When a heavy bituminous crude oil is produced, i.e., a crude oil with an American Petroleum Institute (API) gravity of less than 20 0 API, but more specifically bituminous crude oils in the range of 8 to 12 0 API, it requires blending with a diluent in order to reduce the viscosity to a point that allows transportation by pipeline at ambient temperatures. Often transport by pipeline is only required for a relatively short distance from the producing area, such as is the case for instance for Canadian bitumen transported on gathering pipelines to a nearby railcar loading terminal, or a Colombian heavy crude transported to a port from which it can be exported by ship. From these locations, the heavy crude oil can be transported to its final destination in undiluted or only lightly diluted form by using insulated railcars or ships with reheating capabilities. It will almost always be cost advantageous to remove the diluent in the rail terminal or loading port for reuse in the producing region. Recovering the diluent reduces the volume of product to be transported to the final destination, and usually the diluent has a higher value in the producing region than it has in the markets to which the heavy crude oil is shipped.
Other applications include delivery of the blend of bituminous heavy crude and diluent by long distance pipeline to a refinery that is not well equipped to handle acidic feeds and/or would prefer to remove the light hydrocarbons used as diluent in order not to burden its atmospheric distillation column. In many refineries, once the light hydrocarbon diluent is removed, the bitumen can be directly fed to specialized units designed for upgrading of very heavy fractions, such as cokers or crackers, bypassing the atmospheric and vacuum distillation steps.
The diluent, which typically consists of light hydrocarbons of which pentanes, hexanes and heptanes make up the majority, can be separated from the heavy crude oil by boil-off or simple distillation because of the wide difference in boiling range between the diluent and the heavy bituminous crude oil. The methods to do so are well understood by those skilled in the art, and the installations used for this purpose are known in the industry as Diluent Recovery Units, or DRUs.
Heavy, bituminous crude oils that are likely to be processed in DRUs are often high in naphthenic acids, sulfur, and other contaminants that make the crude oil more difficult to process and cause these crudes to trade at significant discounts to lighter, sweeter crudes. The acidity of crude oils is usually expressed in milligrams of potassium hydroxide needed to neutralize one gram of oil (mg KOH/g), and referred to in the industry as the Total Acid Number (TAN). Crude oils with a TAN of more than 1.0 mg KOH/g are considered problematic in most refineries, and will require special metallurgy, dilution with crude oil with low acidity, or the use of corrosion inhibitors. Many technologies are known to reduce the acidity of crude oils using a wide array of processes, such as destruction through thermal and/or catalytic conversion, adsorption, or extraction. For instance, U.S. Pat. No. 5,820,750A teaches the thermal decomposition of acids whereby the decomposition products such as carbon monoxide, carbon dioxide and water, are continuously removed by a gas sweep.